Iron and zinc are cofactors for hundreds of proteins making them essential for viability. When grown in metal-limiting environments, microorganisms must increase their ability to scavenge these metals to maintain maximal growth rates. As a result, intricate regulatory systems have evolved to up-regulate metal ion acquisition in response to nutritional depletion (Rutherford et al., 2004). In Saccharomyces cerevisiae, iron- and zinc-acquisition are controlled by Aft1p and Zap1p, respectively. These transcriptional activators sense the nutritional status of their respective micronutrients and, in response to deficiency, induce genes involved in uptake. However, some studies indicate that iron- and zinc-uptake systems are inducible under metal replete conditions when Aft1p and Zap1p are believed to be silent. For example, the expression of genes involved in high-affinity iron- and zinc-uptake fluctuate with the phase of the cell cycle (Cho et al., 1998) and seem to be regulated by carbon starvation (Haurie et al., 2003). These studies suggest that iron and zinc bioavailability are not the only environmental stimuli that affect metal accumulation and that Aft1p and Zap1p are not the only regulators of iron and zinc uptake. Therefore, identification of novel genes involved in the regulation of iron and zinc homeostasis is of critical importance for a proper understanding of metal metabolism.
IZH2 expression is induced by both zinc-deficiency via the Zap1p zinc-sensor and zinc-toxicity via the Mga2p hypoxia-sensing transcription factor (Lyons et al., 2004). We present evidence suggesting that Izh2p exerts its effects on FET3 by regulating the activities of four transcription factors with previously unrecognized roles in iron homeostasis—Msn2p, Msn4p, Nrg1p and Nrg2p. Msn2p and Msn4p are stress-responsive transcriptional activators (Smith et al., 1998) while Nrg1p and Nrg2p are carbon source-dependent transcriptional repressors (Berkey et al., 2004). We demonstrate that the Msn2p/Msn4p activators and the Nrg1p/Nrg2p repressors are epistatic with respect to the expression of FET3. Furthermore, we demonstrate that Izh2p dependent-repression requires both cAMP-dependent protein kinase (protein kinase A, PKA) and AMP-dependent protein kinase (AMPK). Since PKA inhibits Msn2p/Msn4p (Smith et al., 1998) and AMPK inhibits Nrg1p/Nrg2p (Kuchin et al., 2002), our findings fit a model in which Izh2p regulates FET3 expression via negative regulation of AMPK, positive regulation PKA or both. It remains to be seen how Izh2p affects PKA and AMPK and whether these transcription factors affect FET3 expression through binding to cis-regulatory elements.
The physiological importance of FET3 regulation by Izh2p is still a mystery. However, Izh2p was recently identified as a cell surface receptor for the plant protein osmotin (Narasimhan et al., 2005). While the exact function of osmotin remains a matter of debate, the fact that it is induced by plants as part of the innate immune response and possesses potent antifungal activity suggests that it functions as a primary line of defense against fungal pathogens (Linthorst, 1993). Therefore, from a pharmacological standpoint, there is significant interest in understanding how osmotin affects fungal physiology. Since yeast with defects in high-affinity iron-uptake show decreased virulence (Eck et al., 1999; Ramanan et al., 2000), the regulation of high affinity iron-uptake represents a reasonable mechanism by which osmotin, via Izh2p, may exert an antifungal effect.
Izh2p belongs to a newly discovered family of receptors known as PAQRs (Progesterone and AdipoQ Receptors) that are ubiquitous in eukaryotes (Tang et al., 2005). The first members of this family to be characterized as receptors were the membrane progesterone receptors from seatrout (Zhu et al., 2003a) and the adiponectin (AdipoQ) receptors from human (Yamauchi et al., 2003). Adiponectin is an insulin-sensitizing hormone and the human adiponectin receptors are believed to play an important role in the etiology of type II diabetes. Despite the medical importance of adiponectin receptors, little is known about how they convert extracellular signals into physiological changes inside cells. Indeed, all that seems to be known is that they somehow transmit signals to AMPK (Kadowaki et al., 2006). In this report, we show heterologous expression of two human adiponectin receptors in yeast. Not only do these receptors repress FET3 in response to adiponectin, this effect requires the same signaling proteins as Izh2p overexpression, including AMPK. The functional expression of these receptors in yeast demonstrates mechanistic conservation in the PAQR receptor family and establishes a valuable model system for the investigation of a pair of receptors that are critical for human health.